Posted By
Flaviosr
on 2021-02-27
15:44:57
|  Plus/4 issues trying to solve with Diag264
Dear All,
I am trying to repair a Plus/4 with a black screen!
I have replaced U13 and U16 (with the help of Diag264 "and of another group, before discovering this one).
Then I replaced U17 (again "with the help of Diag264) but the issue is that now I am stuck... I receive the same error!!!
The chessboard screen is perfect while the other has only some "@" while the rest are horizontal lines.
The back is flashing 7 times with black background.
I have changed different RAM chips in U17 also putting the chips that are working on U13 and U17.
Can somebody help my understand how to go on?
Thank you very much, Flavio
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Posted By
crock
on 2021-02-28
14:56:15
 | Re: Plus/4 issues trying to solve with Diag264
From what you describe, I would be looking for a physical fault such as a dry joint, flaky socket, or solder bridge. The pattern implies D6 is stuck high which, as you say, is facilitated by U17.
If D6 was being pulled high on the whole databus, the CPU would not be able to read ROM's correctly, so it is most likely RAM related, and if you're sure the chip you replaced it with is good, I would be looking for physical defect.
Rob
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Posted By
Flaviosr
on 2021-03-01
15:02:27
| Re: Plus/4 issues trying to solve with Diag264
I checked and I do not see solder bridges or other issus... but I still have s trange behaviour of U17.
I try to describe the issue... let's imagine that the chip is what follows:
1 2 3 4 5 6 7 8
|------------------|
| <
|------------------|
9 0 A B C D E F
Pins 1, 3, 4, 5, 6, 9, 0, A, B, C, E, F are connected between every chip so, for example, all pins 1 give continuity between them (and only between them!)...
On the contrary pins D are not connected...
The only issue I see with U17 is that ALL pins 2 are connected APART pin 2 of U17... perhaps there is a cut line... I could put a wire but I do not where to put it... am I right? If so, from to where?
I have the idea to simply connect pin 2 of U16 with pin 2 of U17 but I am afraid of killing the Plus/4...
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Posted By
Flaviosr
on 2021-03-01
15:03:58
| Re: Plus/4 issues trying to solve with Diag264
Uff... when posted the "image" lost some spaces... I create the "chip" in a different way:
1 2 3 4 5 6 7 8
|------------------|
| ----------------<
|------------------|
9 0 A B C D E F
The tag in on the right side in my "picture"...
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Posted By
Flaviosr
on 2021-03-03
14:53:18
| Re: Plus/4 issues trying to solve with Diag264
Sorry but (as you can read) I am not a technician and I have to learn.
According to your info, probably in my Plus/4 the issue is related to the fact that pin 7 of U17 is not connected? Perhaps a cut trace somewhere (I will check closely) as crock wrote?
Do you think I can "simply" put a wire connecting pin 7 of U17 with pin 7 of U16 (they are now both on socket and it would be easy, without welding)?
Thank you
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Posted By
MCes
on 2021-03-03
15:45:54
 | Re: Plus/4 issues trying to solve with Diag264
During CHIP removing it's possible to damage the PCB traces or the pin holes internal metallization that join electrically front traces with bottom traces.
This case can be solved with a simple wire that join the 2 half-circuit making them a unique circuit again.
This has to be done for each trace that was divided in two part.
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Posted By
Flaviosr
on 2021-03-07
15:22:35
| Re: Plus/4 issues trying to solve with Diag264
I found the cut track... below U16!!! Now the Plus/4 works... thank you for your help.
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Posted By
MCes
on 2021-03-08
06:18:00
| Re: Plus/4 issues trying to solve with Diag264
Great! enjoy your 8bit....
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Posted By
Flaviosr
on 2021-03-21
09:00:36
| Re: Plus/4 issues trying to solve with Diag264
It seems that the repairing is not yet finished.
I finally got the harness to completely check the Plus/4 and I have issues with the cassette port and the user port (I am more concerned with the cassette port).
I tried the "Extended Port Test" but I understand what I do without understanding where there could be an issue... for example in the 6529 part I see:
WRITE: FF => XXXXXXXX
READ: BB => X.XXX.XX
P2 does not change in any way...
Any idea on where there could be an issue...?
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Posted By
crock
on 2021-03-21
12:47:19
| Re: Plus/4 issues trying to solve with Diag264
Did you make the loop backs or yourself or did you buy them? If so, where from?
What is the value of the status byte when the cassette test fails?
The status of the ports as you show in your post are correct for when the dongles are attached, as the motor control pulls the sense line low. eg:
WRITE: FF => XXXXXXXX
READ: BB => X.XXX.XX
So, when you say "it doesn't change in any way", do you mean toggling the state of any of the bits when "writing" to the 6529 has no effect on the read value, or just for bit 2? If just for bit 2, this is expected. If you want to test bit 2, you will need to toggle bit 3 of the CPU port, like this:
WRITE: 00 => ----....
READ: D0 => XX.X....
Then, the 6529 port should show:
WRITE: FF => XXXXXXXX
READ: FF => XXXXXXXX
Then you should be able to toggle bit 2:
WRITE: FB => XXXXX.XX
READ: BB => X.XXX.XX
The "advanced port tests" are exactly as the name implies. It's difficult to use them meaningfully without understanding the plus/4 schematics and hardware architecture.
cheers, Rob
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Posted By
Flaviosr
on 2021-03-21
13:43:27
| Re: Plus/4 issues trying to solve with Diag264
Hello,
I have bought the harness only package from an ebay seller named "cespok_64" from Spain.
He seems professional with a very good range of products for testing the TED family.
The status byte is 7 (I have read from the site about a possible issue on sensing or powering the cassette deck... but I have no idea on where to "put my hands").
I have been able to change the CPU to:
WRITE: 00 => ----....
READ: D0 => XX.X....
BUT I read on the 6529:
WRITE: FF => XXXXXXXX
READ: BB => X.XXX.XX
And if I press 2 nothing changes on READ...
I have written that I have problems also with the user port. I get 44 as status byte... perhaps the two problems are interconnected...
Thank you, Flavio
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Posted By
crock
on 2021-03-21
19:30:16
| Re: Plus/4 issues trying to solve with Diag264
Can you do the same test without the cassette dongle, but keep the user port dongle in place.
You should see that the user port test passes and that you can now toggle bit 2.
Rob
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Posted By
Flaviosr
on 2021-03-23
16:20:13
| Re: Plus/4 issues trying to solve with Diag264
How do you know that?!?!?  Ok... you know it and that's enough.
Exactly as you wrote.
Now the user port is ok and I can toggle bit 2... but I still get an error for the cassette port... is this normal or my cassette port is really "sick"? Or the dongle is "not good"?
Thank you
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Posted By
crock
on 2021-03-23
20:15:18
| Re: Plus/4 issues trying to solve with Diag264
Much as I would like to claim supernatural powers, I'm afraid it's nothing more than experience coupled with a decent knowledge of the hardware.
Your problem is that the cassette is not getting power for the motor. The unregulated 9v power is switched on by a transistor controlled by bit 3 of the CPU. You need to trace that power from the from the power socket, through the switch, fuse, rectifier and transistor. I'd start with the fuse.
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Posted By
Flaviosr
on 2021-03-27
16:48:27
| Re: Plus/4 issues trying to solve with Diag264
I am not so good in checking voltage and if I make a wrong step I could "create" some problems.
I have check the 3 candidates and it seems that the transistor is gone: I measure a voltage between C->B and C->E... perhaps a friend has one spare... a BD243F that should be more strong... I keep you informed.
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Posted By
Flaviosr
on 2021-03-28
14:40:04
| Re: Plus/4 issues trying to solve with Diag264
No results... unwelded the transistor works well and the new transistor started working like the old one...
I have now the doubt that there is something wrong with the connection with the CPU and the transistor... sorry for my lack of knowledge but... what do you mean with bit 3? Pin 3? How can I check the connection between the CPU and the transistor? It is a direct connection?
Thank you for your time
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Posted By
Flaviosr
on 2021-04-11
07:38:06
| Re: Plus/4 issues trying to solve with Diag264
Before writing again I have studied a bit to avoid ignorant sentences...
I have bought an unwelding station to be able to unmount the rectifier and it does work.
I have noticed that the voltage between pin 4 of the 7406 and the transistor does not change when it does between pin 3 of the 7406 and the CPU... so that the CPU is not able to control the transistor the right way.
Could it be that the 7406 is "dead"? What do you think?
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Posted By
crock
on 2021-04-11
10:10:55
| Re: Plus/4 issues trying to solve with Diag264
Yes, a dead 7406 is very possible. Sorry, I should have highlighted that when I first replied. I did not think it likely because if the 7406 was completely failed, many of the other functions would also have failed, but in your case it looks like just the one inverter of the 7406 may have failed.
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Posted By
Flaviosr
on 2021-04-25
10:54:59
| Re: Plus/4 issues trying to solve with Diag264
Finally the "new" 7406 is here but... replacing it did not solve the issue, that is still the same!
I have noticed that there is no continuity between pin 4 of the 7406 and the central pin of the transistor... I have again to admit my ignorance... I have read the schematics where the connection is direct... could it be there is something in the middle not shown or, what I suppose, a broken track (this Plus/4 had another broken track...)?
The issue is that I lose the track when it comes in front below the 8551... I do not know where it is supposed "to go"... can you help?
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